The present invention relates to the formation and manufacturing of diffractive optical input and output grating couplers in surface plasmon resonance (SPR) sensor chips. The SPR sensor chips are being employed in optical bio-/chemical sensor systems, where the function of the SPR sensor chips is to measure bio-/chemical compounds in liquids or in gasses.
In surface plasmon resonance (SPR) sensors, it is important to have an efficient and reliable coupling without critical alignment between the following three components: 1) the light source (LS); 2) the sensing domain (SD), and 3) the optical detector (OD). The SD is defined as the region where interaction between the light and the surface plasmon occurs. It comprises an SPR metal film (typically a few tenths of nanometers) and a superstrate of one or more bio-/chemically active sensing areas. The optical coupling between the LS and the SD can be defined as the input coupling (IC) and the optical coupling between the SD and the optical detector can be defined as the output coupling (OC).
SPR sensors can be divided into three main concepts (A, B and C) according to the integration between the 5 components as defined above, LS, SD, OD, IC and OC. A is the discrete concept, where all 5 components are separated mechanically and the IC and the OC are achieved by means of optical components such as lenses, mirrors, optical fibres, and filters or diffractive optical elements (DOEs). B is the sensor chip concept, where the OD and the IC and OC are integrated in an SPR sensor chip and the optoelectronic components LS and OD are either discrete components and may be mounted on the same electrical circuit board, or they may be integrated on the same optoelectronic chip. C is the integrated optics sensor concept, where all 5 components are integrated on the same chip.
Concept A has traditionally dominated the commercial market of SPR sensors (e.g. products such as BIAcore and IBIS). The disadvantages of concept A are large, bulky and expensive systems, which usually require substantial service. For bio-/chemical sensors, concept C has attracted a great deal of interest within the last 20 years. Typically, an SPR sensor based on this concept consists of optical waveguides as ICs and OCs directing the light from an integrated LS to the SD disposed in a microchannel, and an integrated OD detects the output light beam. Although it is rather a hybrid solution than full integrated one, Texas Instruments has approached this concept and have commercialised an SPR system marketed under the name Spreeta, where all components are integrated in the same housingxe2x80x94see EP 0 797 090. The Spreeta system has a fairly low production cost, but the disadvantages are an unhandy housing and the fact that the user has to dispose all optical and electro-optical components, when replacing the bio-/chemistry on the SD.
The present invention relates to concept B and it has the advantages of the user friendliness of A and the simple optics and low price of C. In fact, since only the sensor chip is being replaced, the production costs can be very low. The present invention is an oblique angle holographic method of formation of ICs and OCs as diffraction gratings integrated on an SPR sensor chip. The formation of ICs and OCs as diffraction gratings is being made in such a manner that the coupling between the LS, the SD and the OD can be established without critical alignment. This enables the light to be accurately directed to the SD with an appropriate angle, focal length and focal point size. The present oblique angle holographic method is particularly suitable for formation of diffraction gratings that have large diffraction angles, as high as xcx9c80xc2x0 from the plane of incidence.
For SPR sensing, the angle of incident light to the SPR film is lying in the range 40xc2x0 to 80xc2x0. One method to couple light into an SPR sensor chip is using a high index prism and an index matching gel (U.S. Pat. No. 5,313,264). In the present invention, where index-matching gels are eliminated, diffraction gratings employed as ICs and OCs are disposed onto an SPR sensor chip.
For an SPR sensor chip with mutually parallel flat topside surface and flat backside surface, the diffraction condition that light rays from each grating spacing of the diffraction grating interfere constructively yields the following expression for the grating spacing ap of the p""th grating element;                                           a            p                    =                                                    m                ⁢                                  xe2x80x83                                ⁢                λ                                                              n                  g                                ⁢                                  x                  p                                                      ⁢                                                            y                  i                                ⁡                                  [                                      1                    +                                                                  (                                                                              x                            p                                                                                y                            i                                                                          )                                            2                                                        ]                                                            1                /                2                                                    ,                            (        1        )            
where xcex is the wavelength of light, m is the diffract on order, xp and yi are the horizontal and vertical distances between the focal point of the diffraction grating and the position of the p""th grating element with p=0 being the first element and p=N being the last element in the grating. In case of one or more reflection points (M) of light between the p""th grating element and the focal point of the diffraction grating, yi has to be multiplied by M+1.
For SPR sensing, the large angle of incidence of the light beam puts high requirements to the accuracy of the formation of the diffraction gratings. This is evident from the following example, where we assume that the input light beam has a wavelength of 670 nm, the substrate of the SPR sensor chip has a refractive index of 1.65 (e.g. a polymer substrate with high refractive index), and the SD comprises an SPR gold film with a superstrate on the top having a refractive index of 1.46 (e.g. a polymer membrane for ion-detection). As a result, the SPR angle is xcx9c73xc2x0. In order to cover this angle in the angle span of the input light beam, the SPR sensor chip may have the following dimensions: yi=2 mm, xp=0=8 mm and xp=N=5 mm resulting in an aperture of 3 mm. Assuming a diffraction order of m=1, eqn. (1) yields a0=418 nm and aN=437 nm. The number of grating periods is xcx9c7050 and the difference in the grating spacing between two neighbouring grating elements is xcx9c(aN-a0)}/7050=0.003 nm. For maximum diffraction efficiency, the depths of the gratings (d) are approximately xcx9c100 nm for reflection gratings and xcx9c800 nm for transmission gratings.
In practice, it would be acceptable to divide the aperture into sections of grating elements with a fixed periodicity in each section. Dividing the full aperture into 100 sections of grating elements, the minimum difference in the grating spacing from one section to the neighbouring section can then be increased to 0.3 nm. However, it is noted that for m=1, the requirement, of the method of formation of the diffraction grating are still very severe.
Alternatively, one can increase the diffraction order m, and the dimensions of the diffraction gratings scale accordingly. On the other hand, by choosing a large value of m, e.g. m greater than 10, it becomes more difficult to optimise the performance of the diffraction grating. There is therefore a need in the art of a method, which accurately forms diffraction gratings on SPR sensor chips working in low order diffraction modes, m less than 10.
There are numerous methods of formation of diffraction gratings. The gratings are either directly fabricated on a substrate, e.g. glass or they are made in a mould, and the structure in the mould is subsequently transferred to another substrate, usually a transparent plastic like acrylics or polycarbonate. There are mechanical methods like single-point diamond turning [P. P. Clark, and C. Londoxc3x1o, Opt. News 15, p. 39-40 (1989)], where a diamond tool with a radius as small as a few micrometers is translated incrementally to grind the desired grating profile into the substrate. A more coarse mechanical method is plunge-cut diamond turning [J. Futhey and M. Fleming, Superzone diffractive lenses, Vol. 9 of 1992 OSA Technical Digest Series, pp. 4-6], where a diamond with a triangular or trapezoidal profile is rotated and transfers the profile to the substrate. The groove dimensions with these methods are limited to the micron range and are most suitable for spherically symmetric structures.
There are scanning analogue writing methods like variable-energy e-beam lithography [E.-B. Kley and B. Schnabel, Proc. SPIE 2640, pp.71-80 (1995)], and laser micromaching [G. P. Behrmann and M. T. Duignan, Appl. Optics 36, pp.4666-4674], where a focussed laser beam writes directly onto the substrate itself or a photoresist. There are phase-only computer generated holograms also known as kinoforms [L. B. Lesem, P. M. Hirsch, and J. A. Jordan Jr., IBM J. Res. Develop. 13, pp.150-155 (1969)], which are fabricated by printing a large-format grey-scale of the desired phase and photoreducing the print into emulsion, which is subsequently developed to generate the desired phase object. Lithographic methods and kinoforms are very flexible methods and they can generate arbitrary diffraction gratings. The draw-back of these methods is the fact that they rely on scanning a lithographic tool with a resolution of typically 50 nm-1000 nm and a positioning accuracy over a large area. With the requirements for the diffraction gratings as mentioned above, the resolutions achievable are not sufficient.
For fabrication of gratings, there are multimasks photolithographic methods [J. D. Mansell, D. R. Neal, and S. W. Smith, Appl. Optics: 36, pp.4644-4647 (1997)] typically with four levels of mask layers to form a binary optic structure as an approximation to the grating profile. There are single masks analogue methods like grey-tone photolithography [U.S. Pat. No. 5,482,800 (Jan. 9, 1996)], where a large array of black spots on a mask creates a desirable diffraction pattern, which under UV exposure generates the desired grating profile on the photoresist. The advantages with the photolithographic methods are that the whole structure is exposed in a short time and thermal drift effects are small. The resolution of photolithographic methods is usually limited to  greater than 20 nm, which also is not sufficiently low for the requirements of the diffraction gratings mentioned above.
Interferometric methods based on producing interference fringes from standing waves [N. K. Sheridan Appl. Phys. Left. 12, pp.316-318 (1968)], transmission fringes from a Fabry-Perot interferometer or from superimposition of a Fourier series of sinusoidal patterns of appropriate amplitude and phase [M. Breidne, S. Johansson, L.-E. Nilson, and H. Ahlen, Opt. Acta 26, 1427 (1979)] have also been reported. Technologically, these methods are difficult and they are limited to particular shapes of diffractive structures.
Analogue holographic recording methods are based on creating an interference pattern on a photosensitive film on a substrate from two beams originating from the same laser [E. B. Champagne, J. Opt. Soc. An. 57, 51 (1967); J. Latta, Appl. Opt. 10, 599 (1971); M. Miler, I. Koudela, and I. Aubrecht, Appl. Opt. 38, pp.3019-3024 (1999)].
The method of the present invention is based on an analogue holographic method, where at least one of the light beams has an oblique angle of incidence onto the substrate and where the interference pattern is transferred to an SPR sensor chip. In addition, the light beams are focussed by means of focussing optics such as lenses and overlap in such a way as to create a light interference pattern on a photosensitive film such as a photoresist; and after developing the photoresist, a surface relief pattern is created and it is transferred to an SPR sensor chip to form an IC. An input light beam incident on the IC is directed and focussed towards the SD at angles covering the SPR angle. Similarly, using the method of the present invention an OC of the SPR sensor chip can be created.
The method of the present invention further includes a procedure of positioning the focal points of the two overlapping light beams in order to reduce optical aberrations up to third order.
In contrast to other methods of forming diffraction gratings, the method of the present invention meets the requirements of accuracy of the formation of the IC and OC of the SPR sensor as mentioned above.
It is an object of the present invention to provide a method of formation of a diffraction grating on an SPR sensor chip as an input optical coupler, which directs and focuses an input light beam onto a sensing domain at an oblique angle defined as an angle larger than 40xc2x0.
It is a further object of the present invention to provide a method of formation of a diffraction grating on an SPR sensor chip as an output optical coupler, which directs and collimates the light beam reflected from the sensing domain to an optical detector.
It is a still further object of the present invention to provide a method of formation of diffraction gratings, where the diffraction grating images a light source with a known angular energy distribution onto a predefined pattern on one surface of an SPR sensor chip.
It is a still further object of the present invention to provide a method of formation of diffraction gratings, where the minimum difference in grating spacing between two grating elements of the diffraction grating is in the nanometer range and even in the subnanometer range.
It is a still further object of the present invention to provide a method of formation of diffraction gratings, where the diffraction grating is created rapidly to avoid excessive effects of thermal instabilities in the recording process.
It is a still further object of the present invention to provide a method of formation of diffraction gratings, where no mechanical scanning is needed that inherently could increase the inaccuracies of the formation of diffraction grating.
It is a still further object of the present invention to provide a method of formation of diffraction gratings by positioning the focal points of the two laser beams at positions such that aberrations are reduced up to third order.
In a first aspect, the present invention relates to a method of forming a first surface relief pattern adapted to be replicated onto a substantially plane surface of a substantially transparent member to form a first diffractive optical element, said substantially transparent member being adapted to form part of a surface plasmon resonance sensor, the method comprising the steps of p1 providing a master substrate, said master substrate having a substantially plane surface,
providing a photosensitive layer of material onto the substantially plane surface of the master substrate,
exposing the photosensitive layer to a first and a second wave of electromagnetic radiation so as to expose the photosensitive layer to a first interference pattern generated by a spatial overlap at an intersection between the first and second waves of electromagnetic radiation, wherein
the first wave of electromagnetic radiation, at the position of the photosensitive layer, has a first mean propagation vector, and wherein
the second wave of electromagnetic radiation, at the position of the photosensitive layer, has a second mean propagation vector, the second mean propagation vector forming an angle to the first mean propagation vector at the intersection between the first an second wave of electromagnetic radiation,
wherein the angle between the first and second mean propagation vectors is selected so as to change direction of propagation of an incoming wave of electromagnetic radiation having an incoming mean propagation vector in such a way that the smallest angle between the incoming mean propagation vector and a diffracted mean propagation vector is larger than 40 degrees when said incoming wave of electromagnetic radiation is incident upon the first diffractive optical element replicated in the substantially plane surface of the substantially transparent member.
Preferably, the method according to the first aspect of the present invention further comprises the steps of
rotating the master substrate approximately 180 degrees around a normal to the surface holding the photosensitive layer,
forming a second surface relief pattern on the substantially plane surface of the master substrate by
exposing the photosensitive layer to the first and second waves of electromagnetic radiation so as to expose the photosensitive layer to a second interference pattern generated by the spatial overlap at the intersection between the first and second waves of electromagnetic radiation, wherein
the first wave of electromagnetic radiation, at the position of the photosensitive layer, has a first mean propagation vector, and wherein
the second wave of electromagnetic radiation, at the position of the photosensitive layer, has a second mean propagation vector, the second mean propagation vector forming an angle to the first mean propagation vector at the intersection between the first and second wave of electromagnetic radiation,
wherein the angle between the first and second m an propagation vectors is selected so as to change direction of propagation of an incoming wave of electromagnetic radiation having an incoming mean propagation vector in such a way that the smallest angle between the incoming mean propagation vector and a diffracted mean propagation vector is larger than 40 degrees when said incoming wave of electromagnetic radiation is incident upon replication of the second surface relief pattern replicated in the substantially plane surface of the substantially transparent member as a second diffactive optical element.
In most practical situations, the first and second waves of electromagnetic radiation have substantially the same wavelength. In fact, the first and second waves of electromagnetic radiation may be emitted by a same light source, which may comprise a laser source, such as a HeCd laser, a Kr-laser, an excimer laser, or a semiconductor laser. The first, second and the incoming waves of electromagnetic radiation are characterized as object wave, reference wave, and reconstruction waves, respectively.
The method according to the first aspect may further comprise the step of developing the photosensitive layer.
The master substrate itself may constitute the substantially transparent member so that the photosensitive layer is provided directly onto the substantially transparent memberxe2x80x94e.g. by spin coating. In this situation the method may further comprise the step of performing a sacrificial-layer-etch of the photosensitive layer in order to replicate the first and said second surface relief pattern(s) into the substantially plane surface of a substantially transparent member. The step of performing a sacrificial-layer-etch of the photosensitive layer may be achieved by means of ion-milling, chemically assisted ion-beam etching or reactive ion etching.
In case the substantially transparent member does not constitute the master substrate, the method according to the first aspect of the present invention may further comprise the step of forming a negative metal master of the first and second surface relief patterns for further replication of said first and second surface relief patterns. Preferably, the metal master is a nickel master.
Further replication may be achieved by replicating the first and second surface relief patterns from the negative metal master using hot embossing. Alternatively, further replication may be achieved by replicating the first and second surface relief patterns from the negative metal master using injection moulding. As a final alternative, the further replication may be achieved by replicating the first and second surface relief patterns from the negative metal master using injection compression moulding. A metal layer may be provided on top of the diffractive optical element(s) replicated from the surface relies pattern(s). Suitable materials are aluminium, gold, silver or the like.
When applying the method according to the first aspect of the present invention, a focal point of the first wave of electromagnetic radiation and a focal point of the second wave of electromagnetic radiation are positioned according to the following procedure
expanding to third order in x the expression of a recording grating spacing defined as,                     a        record            ⁡              (        x        )              =                  λ        record                              sin          ⁡                      (                          θ              1                        )                          +                  sin          ⁡                      (                          θ              2                        )                                ,
where x is a direction perpendicular to the lines of the interference pattern, xcexrecord is a recording wavelength, xcex81 is the angle of incidence of the first wave of electromagnetic radiation and xcex82 is the angle of incidence of the second wave of electromagnetic radiation,
expanding to third order in x the expression of reconstruction grating spacing defined as,             a      read        ⁡          (      x      )        =                    λ        read                              n          g                ⁡                  (                                    sin              ⁡                              (                                  θ                  0                                )                                      -                          sin              ⁡                              (                                  θ                  i                                )                                              )                      .  
where xcexread is a reconstruction vacuum wavelength, ng is a refractive index of the substantially transparent member, xcex8i is the angle of incidence of the incoming wave of electromagnetic radiation and xcex80 is a diffraction angle of a diffracted wave of electromagnetic radiation, and
minimizing the expression
arecord(x)xe2x88x92aread(x)=A0+A1(xxe2x88x92xcentre)+A2(xxe2x88x92xcentre)2+A3(xxe2x88x92xcentre)3, 
with respect to the position of the focal point of the first wave of electromagnetic radiation and the position of the focal point of the second wave of electromagnetic radiation, where xcentre is the centre position of the interference pattern, and A0, A1, A2, and A3 are the differences between the first, second, third and fourth expansion coefficients of arecord(x) and aread(x), respectively.
In a second aspect, the present invention relates to a coupling element for surface plasmon resonance sensors, said coupling element comprising
a diffractive optical element comprising a grating structure having a monotonically increasing spacing in a predetermined direction, the diffractive optical element being adapted to diffract an incoming wave of electromagnetic radiation having a first mean propagation vector into a diffracted wave of electromagnetic radiation having a second mean propagation vector,
wherein the diffractive optical element forms part of a surface of a solid and substantially transparent member, and wherein the direction of propagation defined by the second mean propagation vector is different from the direction of propagation defined by the first mean propagation vector in such a way that the smallest angle between the first and second mean propagation vectors is larger than 40 degrees.
The grating structure may form a transmission grating structure or, alternatively, a reflection grating structure. The diffractive optical element may be adapted to focus an incoming wave of electromagnetic radiation. Alternatively, the diffractive optical element may be adapted to collimate a diverging wave of electromagnetic radiation.
The diffractive optical element may further comprise one or more calibration marks, said one or more calibration marks being areas with missing grating structures.